1. Field of the Invention
The present invention relates to a field sequential liquid crystal display (FS-LCD), and more particularly, to an LCD capable of obtaining desired chromaticity and luminance regardless of a driving current distribution of a light emitting diode (LED).
2. Description of Related Art
A color LCD generally includes a liquid crystal panel having an upper substrate, a lower substrate, and a liquid crystal injected between the upper and lower substrates. The color LCD further includes a driving circuit for driving the liquid crystal panel, and a back-light for providing white light to the liquid crystal. Such an LCD may be mainly classified into a red (R), green (G), blue (B) color filter type or a color field sequential driving type depending on its driving mechanism.
In the color filter type LCD, a single pixel is divided into R, G, and B subpixels, and R, G, and B color filters are respectively arranged in the R, G, and B subpixels. Light is transmitted from a single back-light to the R, G, and B color filters through the liquid crystal allowing a color image to be displayed.
On the other hand, a color FS-LCD includes R, G, and B back-lights that are arranged in a single pixel that is not divided into R, G, and B subpixels. The light of the three primary colors is provided from the R, G, and B back-lights to the single pixel through the liquid crystal so that each of the three primary colors are sequentially displayed in a time-sharing, multiplexed manner, allowing the display of a color image using a residual image effect.
FIG. 1 is a perspective view of a configuration of a typical color FS-LCD.
Referring to FIG. 1, the FS-LCD includes a liquid crystal panel 100 having a lower substrate 101 in which a thin film transistor (TFT) array (not shown) for switching is arranged to be connected to a plurality of gate lines, a plurality of data lines, and a plurality of common lines. The liquid crystal panel also includes an upper substrate 103 in which a common electrode (not shown) is formed to provide a common voltage to the common lines. The liquid crystal panel further includes a liquid crystal (not shown) injected between the upper and lower substrates.
The FS-LCD further includes a gate line driving circuit 110 for providing scan signals to the plurality of gate lines of the liquid crystal panel 100, a data line driving circuit 120 for providing R, G, and B data signals to the data lines, and a back-light system 130 for providing light corresponding to three primary colors, namely, R, G, and B colors, to the liquid crystal panel 100.
The back-light system 130 includes three back-lights 131, 133, and 135 respectively providing R, G, and B light, and a light guide plate 137 providing the R, G, and B light respectively emitted from the R, G, and B back-lights 131, 133, and 135, to the liquid crystal of the liquid crystal panel 100.
Typically, a time interval of a single frame driven at 60 Hz is 16.7 ms ( 1/60 s). When the single frame is divided into three subframes, as is the case for the FS-LCD, each subframe has a time interval of 5.56 ms ( 1/180 s). The time interval of one subframe is short enough to prevent its field change to be perceived by the human eye. Accordingly, the human eye sees the three subframes during the time interval of 16.7 ms as a single frame, resulting in the recognition of a composite color formed by the three primary colors to display the image.
Therefore, the field sequential driving mode may achieve about three times more resolution as the color filter mode for a same-sized panel, increase light efficiency because no color filter is used, and achieve the same color reproduction as a color television set and achieve a high speed of moving picture. However, because the field sequential driving mode divides one frame into three sub-frames, it requires fast operating characteristics. That is, the field sequential driving mode requires a driving frequency of about six times the driving frequency of the color filter driving mode.
In order for the liquid crystal display to obtain the fast operating characteristics, a response speed of the liquid crystal should be fast and a corresponding switching speed for turning the R, G, and B back-lights on and off should also be relatively fast.
FIG. 2 is a schematic diagram of a back-light driving circuit used in the FS-LCD of FIG. 1.
Referring to FIG. 2, a conventional back-light driving circuit includes a back-light 200 including R, G, and B back-lights 201, 203, and 205, for sequentially emitting R, G, and B light, and a driving voltage generator 210 for providing a driving voltage VLED of a same level to the R, G, and B back-lights 201, 203 and 205.
The R back-light 201 includes two R light emitting diodes (RLED1 and RLED2) serially connected for emitting R light. The G back-light 203 includes one G light emitting diode (GLED1) for emitting G light. The B back-light 205 includes two B light emitting diodes (BLED1 and BLED2) connected in parallel for emitting B light.
The driving voltage generator 210 provides the driving voltage (VLED) of the same level to all of the R, G, and B back-lights 201, 203 and 205 forming the back-light 200. The driving voltage (VLED) is provided to an anode electrode of the R light emitting diode (RLED1) in the R back-light 201, to an anode electrode of the G light emitting diode (GLED1) in the G back-light 203, and to anode electrodes of the two B light emitting diodes (BLED1, BLED2) in the B back-light 205.
The conventional back-light driving circuit further includes a luminance adjuster 208 serially connected between the back-lights 201, 203, and 205 and the ground for adjusting luminance of light emitted from the back-light 200. The luminance adjuster 208 includes a first variable resistor (RVR) connected between a cathode electrode of the R light emitting diode (RLED2) in the R back-light 201 and the ground for adjusting luminance of light emitted from the R back-light 201, a second variable resistor (GVR) connected between a cathode electrode of the G light emitting diode (GLED1) in the G back-light 203 and the ground for adjusting luminance of light emitted from the G back-light 203, and a third variable resistor (BVR) connected between the cathode electrodes of the two B light emitting diodes (BLED1, BLED2) in the B back-light 205 and the ground for adjusting luminance of light emitted from the B back-light 205.
Conventionally, although forward driving voltages (RVf, GVf and BVf) of the light emitting diodes (RLED, GLED and BLED) in the R, G and B back-lights 201, 203 and 205 are different from one another, the same driving voltage, for example, 4V is provided to the R, G and B back-lights 201, 203 and 205 from the driving voltage generator 210. For example, the R light emitting diode (RLED) requires a forward driving voltage (RVf) of 2.2 V. The G light emitting diode (GLED) requires a forward driving voltage (GVf) of 3.3 V. The B light emitting diode (BLED) requires a forward driving voltage (BVf) of 3.4 V.
Conventionally, since all the R, G and B back-lights 201, 203 and 205 are provided with the same driving voltage (VLED) of 4V, when trying to drive the R light emitting diodes (RLED1 and RLED2), the R light emitting diodes (RLED1 and RLED2) are applied with a forward driving voltage (RVf) of 2.2V through the first variable resistor (RVR) to adjust luminance of light emitted from the R back-light 201.
When trying to drive the G light emitting diode (GLED1), the G light emitting diode (GLED1) is applied with a forward driving voltage (GVf) of 3.3V through the second variable resistor (GVR) to adjust luminance of light emitted from the G back-light 203. Furthermore, when trying to drive the B light emitting diodes (BLED1 and BLED2), the B light emitting diodes (BLED1 and BLED2) are provided with a forward driving voltage (BVf) of 3.4V through the third variable resistor (BVR) to adjust luminance of light emitted from the B back-light 205.
Therefore, the conventional back-light driving circuit as described above is provided with the same driving voltage of 4V, regardless of whether the R, G and B light emitting diodes are driven with driving voltages that differ from one another. Since the R, G, and B light emitting diodes are provided with the same driving voltage during the three sub-frames of a single frame used to drive the R, G and B light emitting diodes, power consumption is increased. Furthermore, the driving voltage generating circuit according to conventional mechanisms needs to generate a driving voltage that generally corresponds to the largest voltage of the driving voltages required for the R, G and B light emitting diodes.
Another problem is that the forward driving voltages provided to the R, G and B light emitting diodes in each sub-frame need to be manually adjusted using the variable resistors. When distribution of driving currents of the light emitting diodes is large, it is difficult to provide the forward driving voltages suitable for the respective R, G and B light emitting diodes by only manually adjusting them using the variable resistors.